Queen (butterfly)

The queen butterfly oviposits one egg at a time. Each individual egg can be found on leaves, stems, and flower buds of the host.[3] The eggs are usually pale green, but may also be white.[3] It has an ovate conical in shape, a flattened base and slightly truncated top, and is ribbed perpendicularly with raised cross-lines between the ridges.[5][15] Compared to that of the monarch butterfly, the egg of the queen butterfly is taller relative to its width.[7]

A queen caterpillar

Comparatively, the mature queen caterpillar is darker and not as brightly colored as the monarch. It is nearly identical to the caterpillars of Danaus chrysippus.[4]

In the larval stage, the queen is a bluish white, with a reddish-brown underside.[3] It has three pairs of black, fleshy tentacles—one is on the head, one is on the second thoracic segment and one is on the eighth abdominal segment[15][16]—but lack spines.[3] When mature, the caterpillar is brown with purplish prolegs. The caterpillar has been observed in the following transverse stripes: blue, green, yellow, white, and blackish brown.[4] The head is black with white rings.[7] There is no hair on the body of the caterpillar.[15]

Size comparison between a queen caterpillar, a monarch caterpillar and a black swallowtail caterpillar.

The pupa is relatively short and thick, tapering rapidly at the end of the abdomen.[5][15] It is pale green, rarely pale pink, and is frequently ornamented with golden spots.[15] A black transverse band edged with gold is on the abdomen.[3] Below this black abdominal band lies another one in blue.[17] The pupa has very few projections; most notably, it is suspended by a long cremaster from a button of silk.[15] As such, the pupa resembles a pendant.[17]

In general, the pupa of the queen is smaller and more slender than that of the monarch.[7]

As an adult, the queen has two cousins to which it bears a striking resemblance: the more common monarch (Danaus plexippus) and the soldier (Danaus eresimus).

The queen is a moderately large butterfly. It has an average wingspan of 3.1 inches (7.9 cm) to 3.3 inches (8.4 cm). It is easily distinguishable from its cousin, the monarch, by its darker brown ground colour, and lack of stripes decorating the wings.[5][18] The queen bears a closer resemblance to its other cousin, the soldier (Danaus eresimus).[17] It boasts a very tough and flexible chitinous exoskeleton, unlike most other butterflies.[4]

Wing color varies from bright, fulvous brown to rich chocolate, with black marginal bands that are dotted with white or yellow. The underside of the wing is designed much like the upper wing, except it is more pale.[4] The queen has less prominent veins on its wings and lacks the darker, apical shading found in monarchs.[5] The forewing is generally much larger than the rounded hindwing.[15]

Both sexes are morphologically similar. The male's and the female's forewing lengths range from 3.7 centimetres (1.5 in) to 4.6 centimetres (1.8 in), with the mean length equaling 4.2 centimetres (1.7 in).[17] The antennae lack scales.[3] Although all danaids have two pairs of walking legs, the forelegs, the first pair located on the prothoracic segment of the abdomen, is stunted and of little use.[4] The forelegs are more atrophied in the male than in the female.[15] The female uses its short forelegs to scratch the surfaces of leaves to determine which ones are suitable hosts for its eggs.[4] On both sexes, only the atrophied forelegs lack claws.[15]

However, the male queen has a specialized patch of androconia, or a scent-pouch covered in scales, located on its dorsal hindwing.[4][15][17][19] The position and structure of androconia is used to identify different genera. The male also has one reversibly extensible hair-pencil on each side of its abdomen.[7] Hair-pencils, when in contact with these scales, disseminate pheromones near the female at integral stages of successful courtship.[7][8]

The queen larvae feed on Apocynaceae (milkweeds and dogbanes).[5] It can survive on a number of hosts. Common plants include butterflyweed (Asclepias tuberosa) and bloodflower (Asclepias curassavica). In the West Indies, blunt-leaved milkweed (Asclepias amplexicaulis) and honey vine (Cynanchum laeve) are favored. The caterpillar has also been observed on Asclepias nivea, Calotropis procera, and Apocynaceae nerium.[7][17] Other reported host genera include the Apocynum, Gonolobus, Sarcostemma, and Stapelia.[17]

As an adult, its feeding habits are less specific. The butterfly feeds predominantly on nectar from flowers and dead foliage, but can also feed on rotting fruit, sweat, and dry or wet dung, among other substances.[8]

Even as an adult, the queen is drawn to milkweeds (Apocynaceae).[5] However, the butterfly is also attracted to the Nerium, Funastrum, Vincetoxicum, Philabertia, Stapelia.[4]

In addition to the above food sources, males are attracted to Heliotropium, Eupatorium, Senecio, and Crotalaria, plants known to contain the alkaloid lycopsamine. The alkaloid and other precursor compounds from these plants are used to create pheromones used to attract mates.[3] Pheromone precursors are predominantly obtained from Boraginaceae, Asteraceae, and Fabaceae.[19]

For quite some time, the queen had been regarded as highly unpalatable to its vertebrate (mainly avian) predators. This is due to the fact that the queen, like its cousin the monarch, feeds largely on Asclepiads. As the queen and the monarch are closely related, it was assumed that the queen also possesses the ability to effectively sequester and store cardenolides present in milkweeds.[9] As such, the queen and the Florida viceroy was long regarded a classic model-mimic example of Batesian mimicry, similar to the relationship exhibited by the monarch and the viceroy.[10]

However, the unexpected failure of birds to reject successive queens in an experimental setting[10] called into question the legitimacy of this relationship. In fact, experimental evidence suggested that Florida viceroys could be significantly more unpalatable than representative queens. Because experimental evidence showed sampled queens were significantly less distasteful than viceroys, it was purported that Florida viceroys and queens were Müllerian co-mimics.[21] Furthermore, evidence from this study led to the hypothesis that the queen actually enjoys an asymmetric mimicry relationship, gaining an advantage from flying in the company of the relatively more unpalatable viceroy.[21]

Further experimentation suggested that chemical defense of queens is highly labile. It was shown that queens reared on the high-cardenolide A. curassavica sequester and store levels of cardenolides similar to those found in monarchs.[9] These butterflies were regarded as very distasteful and were largely rejected by avian predators. Furthermore, those that were eaten elicited high rates of distress behavior. However, queens reared on S. clausum, a larval host plant known to be a very poor cardenolide source, contain no detectable cardenolide and are essentially palatable to predators.[20] These highly variable responses of avian predators to queens reared on different plants suggest the existence of a food-plant-related palatability spectrum in Florida queen butterflies.[9]

Micro-geographic differences in the environment lead to variation in the dynamics of mimetic relationships even at a local level.[22] Spatiotemporal variation throughout different areas lead to large differences in unpalatability of queens separated by only a few kilometers. This extensive variation supports the idea that automimicy occurs at the intrapopulation level – palatable queens mimic individuals that have higher cardenolide content. By extension, interspecific mimicry is also highly variable. At hydric inland sites, which contain large numbers of A. curassavica, queens and viceroys are distasteful Müllerian mimics of one another, while at coastal sites queens probably serve as the palatable Batesian mimics of viceroys.[22]

Queen unpalatability does not directly mirror either food plant or butterfly cardenolide content.[9] Evidence suggests that the interaction of cardenolides and noncardenolides are utilized for chemical defenses in milkweed butterflies.

Wild queens that fed upon S. clausum as larvae but had access to adult-obtained compounds, such as the pyrrolizidine alkaloids (PAs) used for pheromone production, were observed to be significantly less palatable to avian predators than butterflies without chemical defenses.[20] As such, these alkaloids, which are known to deter spider predators, may make a substantial contribution to queen distastefulness.

During courtship, which occurs while both butterflies are in flight, the male everts his hair-pencils and brushes them against the female's antennae. This act is called "hair-pencilling." The secretion associated with these hair-pencils plays an important role in seducing the female.[23] When the female comes to rest, the male hovers closely above her and subjects her to further “hair-pencilling” before alighting next to the female and copulating with her.[24] Afterwards, the two engage in a postnuptial flight - the male flies with the female dangling beneath him.[3]

The chemicals that comprise the pheromone are secreted by trichogen cells, which are located at the base of each hair-pencil. This liquid secretion moves from these cells, through the cuticle of the hairs, to coat the numerous free, cuticular dust particles that adhere to the hair-pencil surface.[25] Two of the chemicals that comprise this secretion have been identified – a crystalline pyrrolizidinone (ketone) and a viscous terpenoid alcohol (diol).[12] The diol imparts a stickiness that allows the secretion to stay on the dust, and the dust on antennae. The ketone is a releaser pheromone, inducing females to mate. Although insufficient levels of ketone present in the dust particle correlates to lower seductive capacity in the male,[24] some males with low levels of ketone – and even some without hair-pencils – have been known to mate successfully with females. This suggests that although hair-pencil pheromones are of major importance, they are not absolutely essential to mating.[11]

Many butterflies possess extrusible brushlike structures, called hair-pencils. In the queen, the hair-pencils, which are present in the posterior abdomen in the male, are tucked away when the male is not interacting with the female.[24] As such, these organs are thought to serve as important tools for pheromone dissemination during courtship.[24]

Hair-pencils play an important role in courtship success. Although the lack of hair-pencils does not affect the rate at and enthusiasm with which males pursue females, males without hair-pencils experience significantly lower success in achieving copulation. Male queen butterflies with physically normal but chemically deficient hair-pencils also suffer from lower mating success.[11] In addition, adult female queens whose antennae have been blocked are not receptive to advances from competent male queens. However, physical contact between the male's hair-pencil and the female's antennae does not affect a male's mating success.[11] Males without hair-pencils are no less fertile than males with hair-pencils.[11]

That actively hair-pencilling males emit a very definite odor that can even be perceived by humans also supports the idea that it is not the hair-pencil itself that is important in courtship, but rather, the pheromone which the hair-pencil transports.[11]